1. Technical Field
The present invention relates to an information processing apparatus, an information processing method, a recording medium, and a program. More particularly, the invention relates to an information processing apparatus, an information processing method, a recording medium, and a program for reducing the level of transmission power during wireless communication.
2. Background Art
The demand for wireless LANs (Local Area Networks) has grown appreciably in recent years. Wireless LANs are set up extensively today in the office, in households and at so-called hot spots, implemented typically in accordance with the IEEE 802.11b standard using the 2.4 GHz band or with the IEEE 802.11a using the 5 GHz band. The hot spot is a location furnished with a wireless LAN that allows users of information processing apparatuses with wireless communication capabilities (e.g., PDA (Personal Digital (Data) Assistants) or personal computers) on the move to access the Internet. Within about a 100-meter radius of a base station constituting a hot spot, a user carrying around a PDA or a personal computer may communicate wirelessly with the base station for high-speed access to the Internet wired to that base station.
The IEEE 802.11b standard stipulates the use of coding technology known as CCK (Complimentary Code Keying), adopts direct diffusion for the modulation system, and allows for data transmission speeds of up to 11 Mbps. The IEEE 802.11a standard adopts OFDM (Orthogonal Frequency Division Multiplex) for the modulation system and allows for data transmission speeds of up to 54 Mbps.
The basic access procedure according to IEEE 802.11 is called DCF (Distributed Coordination Function) used in what is known as the CSMA/CA scheme to provide autonomous distributed access control. In basic operations under the CSMA/CA scheme, a wireless station attempting to transmit signals verifies the use status of wireless channels in advance (called a carrier-sense operation) to avert interference with signal transmission by other wireless stations. The transmitting station transmits data immediately if there is an unused (i.e., idle) channel available, or postpones data transmission if all channels are being used (i.e., busy) until a channel becomes available. The receiving side returns an acknowledgement (ACK) for confirmation following receipt of the data. If acknowledgement is not returned, the transmitting side retransmits the data.
The IEEE 802.11 standard stipulates the specifications for physical layers of media as well as for MAC (Media Access Control) layers for establishing basic communications using the physical media (e.g., see Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, ANSI/IEEE Std 802.11, 1999 Edition).
Where battery-powered small information processing apparatuses such as PDAs are used as wireless LAN terminals, it is particularly desirable that each apparatus minimize its power dissipation during wireless communication.
If the opposite party of wireless communication (e.g., base station) is sufficiently near, each terminal may lower its level of transmission power in proportion to the distance to the communicating party.
Reducing the level of transmission power is allowed if the communicating party is the base station and if no other terminal is located within the service area of that base station. If any other terminal does exist in the same service area, there can be interference with data transmission by that terminal.
A conventional wireless communication system is described below with reference to FIG. 1. Terminals 1 and 2 within a service area 11 of a base station 3 communicate wirelessly with that base station 3. The base station 3 is wired to the Internet 4. The terminals 1 and 2 can gain access to a server 5 on the Internet 4 via the base station 3.
The terminal 1 is allowed to enter power-down mode for power-saving data transmission in proportion to the distance to the base station 3. The terminal 2 is not designed to have power-down mode and always transmits data at full power to the base station 3. The terminal 1 in power-down mode can transmit data over a distance approximately the same as that between the terminal 1 and the base station 3, i.e., over a range 12 as indicated in FIG. 1. If any other terminal (e.g., terminal 2) is not located within the range 12, that terminal is incapable of successfully performing a carrier-sense operation on the data being transmitted by the terminal 1.
Suppose that during data transmission by the terminal 1 in power-down mode to the base station 3, the terminal 2 outside the range 12 carries out a carrier-sense operation preparatory to its own data transmission to the base station 3, as shown in FIG. 2. In that case, the terminal 2 cannot sense the data being transmitted by the terminal 1 in power-down mode. The terminal 2 then determines mistakenly that the base station is idle and starts transmitting data packets at full power to the base station 3. The data transmission from the terminal 2 disrupts the ongoing reception by the base station 3 of data packets from the terminal 1.
The interference with data reception caused by the deficient functioning of the carrier sense feature has been known as the hidden-terminal problem. The phenomenon can occur not only when the terminals 1 and 2 and the base station 3 are distanced from one another as shown in FIG. 1, but also when obstructions exist between the terminals or between a terminal and the base station.
The IEEE 802.11 standard stipulates that RTS (Request To Send) and CTS (Clear To Send) frames be used to resolve the hidden-terminal problem. What follows is a description of how RTS and CTS frames are typically transmitted and received.
Prior to data transmission, the terminal 1 checks the data length of the data to be transmitted, calculates based on the checked data length a busy time in effect from the time an RTS frame is transmitted to the base station 3 until an ACK frame is received from the base station 3 upon completion of the data transmission to the base station 3, and transmits an RTS frame including the busy time information to the base station 3.
Upon normal receipt of the RTS frame from the terminal 1, the base station 3 transmits within the service area 11 a CTS frame including information about the busy time calculated to be in effect from the time the RTS frame is normally received from the terminal 1 until an ACK frame is sent out following the reception of data from the terminal 1 subsequent to the self-executed CTS transmission. On receiving the CTS frame, the terminal 2 recognizes the busy time for communication between the base station 3 and the terminal 1 and sets accordingly a data transmission-prohibited interval. After normally receiving the CTS frame, the terminal 1 transmits data to the base station 3. Upon normal receipt of data packets from the terminal 1, the base station 3 returns an ACK frame.
The busy time information included in RTS and CTS frames denotes the time calculated to be in effect before an ACK frame is transmitted by the base station 3. The terminal 2 or any other terminal that could potentially become a hidden terminal recognizes the busy time of the available transmission channel upon receipt of an RTS or CTS frame and takes measures to stop transmitting data packets during that time interval so as to avoid collision with transmitted data.
However, there are cases such as one shown in FIG. 3 in which, when the terminal 1 transmits an RTS frame at low power to the base station 3, the terminal 2 does not recognize the start of data transmission and reception between the terminal 1 and the base station 3 until a CTS frame is received from the base station 3. In that case, the terminal 2 determines mistakenly that the base station 3 is idle and starts transmitting an RTS frame or data packets at full power to the base station 3. This can cause interference with the data communication between the terminal 1 and the base station 3.
That is, even if RTS and CTS frames are used, data communication between the base station 3 and the terminal 1 in low power mode can be disrupted by the transmission of data packets or an RTS frame at full power by the terminal 2.
As described above, a problem persists in a wireless LAN system having the service area 11 in which coexist a plurality of terminals including one (e.g., terminal 2) always transmitting data at full power and others (e.g., terminal 1) capable of transmitting data in power-down mode. In such a LAN system setup, data transmission by the terminal 1 in power-down mode is liable to be disrupted by the terminal 2 always transmitting data at full power.